Pulse transformers for propagating pulses with rapid rise times and fall times are well known, and are described, for example, in the text Pulse, digital and switching waveforms, by Milman & Taub, published in 1965 by McGraw-Hill Publishing Company. Pulse transformers utilizing conventional cores and winding techniques, may not provide the necessary coupling and low leakage characteristics required for ultra narrow pulsewidth and high frequency operation with high voltage isolation, as for example isolation to voltages exceeding 10 kV. The difficulty in achieving good coupling and low leakage inductance is compounded by the additional insulation required to stand off the voltage between the transformer primary side at ground potential and the secondary side at the high voltage potential.
To optimize a transformer for passing low distortion rectangular electrical pulse shapes (pulses with fast rise and fall times at relatively constant amplitude), the transformer needs to have low values of leakage inductance and distributed capacitance, together with high open-circuit inductance. Good transient response is needed to maintain the pulse shape at the secondary winding(s) because slow rise times tend to cause switching losses in power transistors and excessive leakage inductance can generate transient ringing.
Leakage inductance is caused by the imperfect coupling of the primary and secondary windings, which in turn generates a leakage flux which does not link with all turns of the windings. The leakage flux acts as another magnetic component, storing and discharging magnetic energy with each frequency cycle of the electrical signal. The leakage flux acts as an inductor in series with each of the primary and secondary windings. This series inductive reactance then causes a frequency sensitive voltage drop (voltage reduction) that increases with frequency, hence constitutes a severe detriment to high-frequency, wide-bandwidth capability.
The physical design of the magnetic core and of the windings of a pulse transformer contribute to the leakage inductance. For high voltage applications, high insulation resistance and high breakdown voltage are required, and in general require even more separation of the windings, which potentially allows more leakage flux to occur. The more the exposure of the windings outside of the core's magnetic flux circuit and the less the proximity of the primary windings to the secondary windings, the more potential exists for leakage flux and the resultant series inductance.
In general, attempting to achieve low leakage inductance is addressed in the prior art by using either flat wide winding materials with minimal insulation or by using interleaved and twisted windings otherwise known as bifilar windings. For windings with a large number of turns, another method is to sectionalize or break up the winding into smaller alternating sections between primary and secondary windings. Neither approach achieves the required coupling since either the magnetic circuit is not sufficiently enveloped or the number of turns is too low for sectionalizing.
The existing design of twisting the primary and secondary windings together in a bifilar fashion on a toroidal magnetic core, as described for example by Milman & Taub, may compromise system performance, as the coupling may be less than desired, and leakage inductance may be excessive for the required performance at the frequencies and pulse shapes required.
Improved pulse transformers are desired.